1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
A conventional communication system uses one or more access nodes to provide network connectivity to one or more mobile nodes. The access nodes may be referred to as access points, access networks, base stations, base station routers, cells, femtocells, pico-cells, and the like. For example, in a cellular communication system that operates according to Universal Mobile Telecommunication Services (UMTS) standards, one or more nodes may be used to provide wireless network connectivity to mobile nodes. The mobile nodes may include cellular telephones, personal data assistants, smart phones, text messaging devices, Global Positioning Systems, navigation systems, network interface cards, notebook computers, desktop computers, and the like. Numerous types and generations of wireless communication systems have been developed and deployed to provide network connectivity to mobile nodes. Exemplary wireless communication systems include systems that provide wireless connectivity to micro cells (e.g., systems that provide wireless connectivity according to the IEEE 802.11, WEE 802.15, or standards) and systems that provide wireless connectivity to macro cells (e.g., systems that operate according to the Third Generation Partnership Project standards—3GPP, 3GPP2—and/or systems operate according to the IEEE 802.16 and IEEE 802.20 standards). Multiple generations of these systems have been deployed including Second Generation (2G), Third Generation (3G), and Forth Generation (4G).
The coverage provided by different service providers in a heterogeneous communication system may intersect and/or overlap. For example, a wireless access node for a wireless local area network may provide network connectivity to mobile nodes in a micro cell or pico-cell associated with a coffee shop that is within the macro cell coverage area associated with a base station of a cellular communication system. For another example, cellular telephone coverage from multiple service providers may overlap and mobile nodes may therefore be able to access the wireless communication system using different generations of radio access technologies, e.g., when one service provider implements a 3G system and another service provider implements a 4G system. For yet another example, a single service provider may provide coverage using overlaying radio access technologies, e.g., when the service provider has deployed a 3G system and is in the process of incrementally upgrading to a 4G system.
The network can instruct access nodes to hand off, e.g., from a macrocell to an overlying microcell, even when the access node detects a stronger signal from the macrocell than the microcell. This technique is referred to as cell expansion because it effectively expands the range of the microcell by applying a bias to the user equipment. Research has demonstrated that cell expansion in a mixed micro/macro cell environment may have a number of advantages. For example, transferring or handing off user equipment (UE) from the macrocells to the microcells increases the number of UE served by microcells and may lead to splitting gains that can enhance the overall capacity of the heterogeneous network. Cell selection enhancements may be implemented in order to improve the cell association to optimize the performance in a system with non-uniform coverage resulting from the very different downlink power transmitted by the macrocells and the microcells. Maximizing the gains from cell expansion in microcells may therefore be a valuable tool for increasing the spectrum available to operators of heterogeneous networks.
Modeling of the performance gains from cell expansion typically assumes that the macrocells and the microcells do not mutually interfere. However, interference between data channels and control channels is expected in actual deployments of heterogeneous networks. For example, interference between control channels used in overlapping macrocells and microcells can lead to an increase in outages due to control channel failures. Studies have suggested that outages due to Physical Downlink Control Channel (PDCCH) failure can increase in microcells when cell expansion is used for the microcells. The effect of interference between the overlapping cells is exacerbated in co-channel and overlap deployments because the control channels for the macrocells and microcells share the same frequencies and/or timeslots in the overlapping regions. Control channels in this kind of heterogeneous deployment would experience significant mutual interference which can lead to control channel decoding failures and outages. Unfortunately, widespread co-channel deployments are a likely (and perhaps inevitable) response to the growth in bandwidth intensive applications that already stretch the capacity of existing networks.